Redoing Plumbing in Old House - Manifold and Tee Question

I'm in the Kansas for the weekend visiting family and my dad was talking about redoing the plumbing in his basement. I thought I would post his situation here to get some advice.

The house was built in 1933 and was originally all galvanized. Over the years, parts have been replaced with a mixture of galvanized and copper. It looks like each time there was a need to change/reroute or add a plumbing fixture - whoever did the works chose the easiest (not always correct) way to do it. There are tees to nowhere, soft copper that has been rolled over as a stop, runs that go from 3/4 galvanized to 1/2" copper and then back to 3/4 galvanized. 3/4 lines teed off of 1/2 lines. Plus, it looks like the pex spaghetti jobs I've seen in the past - just in galvanized and copper. In short, it's a mess. Lots of pressure/temperature drops.

My dad's plan is to take some time this fall and redo and clean up what he can. If he could, he would just start over - but there are two longer trunk/branches that run from the basement to the second story behind tile walls he doesn't want to have to rip out. There's also lots of good copper that doesn't need to be fully redone either.

Most of the 1/2 copper lines in the house run back to the basement. It would be easy enough to cut out the bad/unnecessary piping and tie the existing copper back to a manifold. However, there are two second story bathrooms that have new 3/4 copper running directly from the basement. These have branch manifolds at the bathroom feeding the fixtures. There is no reason to replace these...I hope.

So finally - my question.

On the hot water side, if we put a manifold after the water heater to feed the 1/2 lines for individual fixtures, should we tee off before the manifold to feed to the two 3/4 lines that run to the second story? Or would it be better to continue through the manifold and tee afterwards? Or does it even matter?

I've attached two pictures to illustrate the ideas.

The cold side would essentially work the same way. Manifold that ties into the existing 1/2 lines and then tie into the existing 3/4 cold line to the second story.

...if we put a manifold after the water heater to feed the 1/2 lines for individual fixtures, should we tee off before the manifold to feed to the two 3/4 lines that run to the second story? Or would it be better to continue through the manifold and tee afterwards? Or does it even matter?

Click to expand...

Depending up available pressure and the aize of the incoming line, it might not make any difference. But since the 3/4" would need less pressure to have as much flow as a 1/2" line, I believe, I would likely connect the 3/4" lines after the manifold.

Ideally, I think all 3/4" lines and the manifold (at least 3/4") would branch off the 1" supply line, then 1/2" lines off 3/4" since any two 1/2" lines open at the same time could call for more than a 1/2" supply (manifold) could provide. The lines in our old house were not sized and arranged or ordered properly when it was built, and our washing machine (calling for cold water only) now robs supply from the water heater and thereby turns the shower cold at the far end of the house. So, imagine all lines open at once and try to do things so your most-important fixtures do not have the weakest supply.

Yeah, that's what I was thinking too. I guess the part that I'm unsure about is should I continue the 1" line after the cold water manifold to the water heater?

The inlet and outlet to the water heater are standard 3/4". So, at best I'd have 1" to the water heater and 3/4" afterwards. For some reason, I always thought I needed to keep the hot and cold sides 'balanced'. Do I benefit at all from leaving the water heater and going up a size to 1"? That seems counterintuitive to me but I'm not sure.

Someone who knows more than I will have to tell you about balancing, but feeding the water heater's 3/4" line off the incoming 1" line assures availability there...and then I would be inclined to supply cold water to everywhere else after that. By supplying the water heater first, flushing a toilet being fed on down the line will simply reduce overall pressure before (or maybe even without) actually taking specific flow away from the water heater.

I guess the part that I'm unsure about is should I continue the 1" line after...the water heater?

Yes, and try to keep it that way (since you already have 3/4" going to the second floor), and then Tee your 1/2" lines off the 3/4". Just think of a tree and its branches: All branches get progressively smaller away from the trunk, never larger. But if it is a long run over to the 3/4'" lines going up to the second-floor bathroom and you might be able to spend a little less for new material, you will likely not hurt anything if you pretend those lines are only 1/2" and run 1/2" over to them from the 3/4"...and I suspect a real plumber will speak up if I am wrong about that!